1. Field of the Invention
The present invention relates to a process for producing a fluorinated polymer having ion-exchange groups. More particularly, it relates to an improved process for producing a fluorinated polymer having carboxylic acid type cation exchange groups by a copolymerization in an aqueous medium.
2. Description of the Prior Arts
It has been known that a cation exchange membrane of a carboxylic acid type fluorinated polymer is used as a membrane for partitioning in a membrane type electrolysis of an aqueous solution of an alkali metal chloride to produce an alkali metal hydroxide having high purity and to attain an operation at high current efficiency and high current density and also to produce an alkali metal hydroxide at high concentration in a cathode compartment, for example, excellent characteristics for providing a current efficiency of greater than 90% even though a concentration of the resulting sodium hydroxide is higher than 40%.
It has been proposed to produce a fluorinated polymer having carboxylic acid type cation exchange groups which has high ion exchange capacity and remarkably high molecular weight by a copolymerization in an aqueous medium at a ratio of the aqueous medium to the carboxylic acid type monomer of less than 20:1 under a pressure of higher than 7 kg./cm.sup.2. This is disclosed in U.S. Pat. No. 4,138,373 and U.K. Pat. No. 1,552,060 and Japanese Unexamined Patent Publication No. 49090/1978.
The inventors have studied various copolymerizations of a carboxylic acid type monomer and a fluorinated olefin such as tetrafluoroethylene in an aqueous medium and have found the following interest facts.
It has been found that the molecular weights of the copolymers can be controlled in a wide range by polymerizations in an aqueous medium with a water soluble organic solvent such as alcohols or an aliphatic or alicyclic alkane such as ethane and cyclohexane. In accordance with the control of the molecular weight by said incorporation, the reduction of the polymerization velocity is only small and a reproducibility for molecular weight is remarkably high and the stability of the resulting latex is not lost.
When an alkane is incorporated as a molecular weight regulator, the following effect is found. The deterioration of the monomers in the copolymerization in an aqueous medium can be reduced and the copolymerization velocity can be increased. Since the deterioration of the monomers in the copolymerization is reduced, the efficiency in utilization of the carboxylic acid type monomer is improved to increase the yield of the object copolymer and undesirable effects on the characteristics of the object copolymer is prevented.